Shear wave elastrography is a burgeoning technique which provides mechanical properties of soft human tissue through noninvasively diagnostic ultrasound testing. Its applications include diagnostics in breast, prostate, liver, kidney, thyroid, heart, and blood vessels. Several different methods have been developed for shear wave elastography, for example, shear wave elasticity imaging (SWEI). In SWEI, an acoustic radiation force (push) is applied to soft tissue for a short duration, and the transient shear wave displacement response in the tissue is measured using high framerate ultrasound. Other methods for shear wave elastography include acoustic radiation force imaging (ARFI), supersonic shear imaging (SSI), spatially modulated ultrasound radiation force (SMURF), crawling wave spectroscopy (CWS), and comb-push ultrasound shear elastography (CUSE).
Among the above introduced shear wave elastography methods, SWEI, SSI, SMURF, CWS, and CUSE are quantitative methods, which provide shear wave speed values of the tissue locally. Conventionally, the local shear wave speed is estimated using a time-of-flight algorithm based on a one-dimensional (1 D) correlation method. To estimate the shear elasticity of a spatial point, the time-of-flight estimation is obtained by applying a one-dimensional cross-correlation of the shear waveform measured at two spatial points located at both lateral sides of an estimation point. These two spatial points have the same depth as the estimation point, but have different lateral distances away from the push beam. The shear wave cs is calculated by dividing the distance between the two lateral points by the time of flight, then the shear elasticity value is obtained using μ=ρcs2, where ρ is the density, and μ is the shear elasticity.
Although the time-of-flight method provides a useful estimate of the shear elasticity, it has some drawbacks, and there is significant room for improvement. First, the time-of-flight method inherently assumes the shear wave propagates as a plane wave. However, this assumption is not accurate as shear wave diffraction is not considered in this plane wave model. The shear waves are induced by the acoustic radiation force and are diffracted away from the source. Shear wave propagation is poorly described by simple plane wave propagation. Moreover, this method considers that the tissue is purely elastic and ignores the viscoelasticity of the tissue. This assumption is also not accurate as several studies have proved that the tissue is viscoelastic. These two inaccurate assumptions lead to inaccurate estimates of the shear elasticity value. Furthermore, the shear viscosity value of the tissue is another important tissue parameter that should be useful in diagnostic applications. It would be useful to develop a method to quantitatively estimate the shear viscosity value of soft tissue. So, based on the above backgrounds, the first motivation of this disclosure is to examine whether the time-of-flight method provides accurate parameter estimation in viscoelastic tissue, while the second motivation of the disclosure is to develop a method which provides more accurate estimates of the shear elasticity and shear viscosity values.
This section provides background information related to the present disclosure which is not necessarily prior art.